There is a demand for structures that can be evaluated by a critical dimension scanning electron microscope (CDSEM) (or even atomic force microscopy (AFM) to some extent) on a wafer and still be verified on the mask with today's mask metrology. However, this is only possible if the dimensions and the appearance of the structures are compatible with current CDSEM measurement approaches and if the relevant reference layer can be exposed and dimensions of the underlying layer can be assessed during the measurement of the current layer. Such evaluations are particularly problematic for interconnects due to limited visibility of buried layers. Currently, for 45 nm and below technology nodes there are micro-blossom targets or process control images (PCIs) for reticle registration measurement. However, micro-blossom targets cannot provide 2D registration measurements or provide the wafer CDSEM overly measurement and, at the same time, enable monitoring of interconnections.
A need therefore exists for methodology enabling 2D registration measurement or overlay reticle measurement and monitoring of interconnections and SEM overlay measurements and the devices used for such methodology.